Composite wire drawing die

ABSTRACT

Composite wire drawing die construction is described in which a centrally-located mass of diamond, cubic boron nitride or a polycrystalline mixture thereof defines at least the throat of the wire drawing hole, the mass being flanked or girded by at least one mass of metal bonded carbide that is directly bonded thereto. The composite is readily ground into the form of a solid of revolution. In the preferred construction (for dies for drawing 0.008 inches diameter wire and larger) a composite assembly includes at least one high strength metal ring that is press fitted around a composite body or sub-assembly comprising a polycrystalline mass of diamond girded by a metal-bonded carbide jacket.

United States Patent [191 Wentorf, Jr. et al.

[ Aug. 27, 1974 COMPOSITE WIRE DRAWING DIE [75] Inventors: Robert H. Wentorf, Jr.,

Schenectady; William A. Rocco, v Scotia, both of NY.

[73] Assignee: General Electric Company,

Schenectady, NY.

[22] Filed: Mar. 26, 1973 [21] Appl. No.: 345,178

52 US. c|..; 72/467, 76/107 A [51] Int. Cl. B2lc 3/00 [58] Field of Search 72/467; 76/107 A [56] References Cited UNITED STATES PATENTS 1,935,821 11/1933 Simons 76/107 A 1,944,758 1/1934 Roux 76/107 A 2,058,110 10/1936 SghLoter 72/467 2,689,641 /1954 Lips 76/107 A 2,866,364 12/1958 Bieberich 76/107 A Primary Examiner-Charles W. Lanham Assistant Examiner-Robert M. Rogers Attorney, Agent, or Firm-Leo I. MaLossi; Joseph T. Cohen; Jerome C. Squillaro ABSTRACT dies for drawing 0.008 inches diameter wire and larger) a composite assembly includes at least one high strength metal ring that is press fitted around a composite body or sub-assembly comprising a polycrystalline mass of diamond girded by a metal-bonded carbide jacket.

10 Clairm, 6 Drawing Figures PATENTED A1182 11914 ah /O 6 M COMPOSITE WIRE DRAWING DIE BACKGROUND OF THE INVENTION A polycrystalline diamond wire drawing die construction is disclosed in U.S. Pat. No. 3,407,445 Strong. Composite tool inserts in which polycrystalline diamond is directly bonded to metal bonded carbide are described in U.S. Pat. application Ser. No. 212,408 Wentorf, Jr. et al (now U.S. Pat. No. 3,745,623), filed Dec. 27, 1971. Tool insert construction in which polycrystalline cubic boron nitride (CBN) is directly bonded to metal bonded carbide is disclosed in U.S. Pat. application, Ser. No. 158,711 Wentorf, Jr. et al (now U.S. Pat. No. 3,743,489), filed July l, 1971. Each of these two patent applications are assigned to the assignee of the instant invention and are incorporated by reference.

Single crystal diamond dies are preferred for drawing fine tungsten wire, but for larger wire diameters, above about 0.008 inch, single crystal diamond dies are large, expensive and easily broken. Metal bonded carbide dies are, therefore, generally used for the drawing of tungsten wire in sizes of 0.0 inch and greater, despite the relatively short useful life of such dies.

Diamond wire drawing die assemblies in common use are made of natural single crystal diamond supported in proprietary sintered metal matrices. Support for the diamond crystal die is obtained by locating the diamond in a mounting ring with the space between the diamond and the ring being filled with sinterable metal and then sintering the metal. Sinterable metals useful for this purpose must not attack the diamond during the sintering operation. This criterion automatically rules out the use of high yield strength, high modulus of elasticity metals, because such metals are characteristically strong carbide formers and will, of course, attack a carbon source (the diamond in contact therewith) at the elevated temperatures required for the sintering.

Thus, the inherent nature of the sinterable mounting metal (low modulus of elasticity, low yield strength) prevent the application of significant (greater than about 10,000 psi) compressive support to the outer surface of a single diamond die. Even though at room temperature prior art sintered metal supported diamond dies may be prestressed to about 10,000 psi, this application of compressive stress rapidly diminishes as the operating temperature of the die is raised, if the wire drawing operation is to be conducted hot.

Diamond is weak in tension and it would be particularly beneficial to be able to offset this deficiency by permanently applying a compressive stress of greater than about 10,000 psi to the diamond outer surface.

Polycrystalline diamond wire drawing dies such as disclosed in U.S. Pat. No. 2,407,445 present the same problem insofar as the mounting of the die is concerned. If, instead of using the sintered metal matrices, one were to attempt to press fit such a die into a binding ring, the expense of grinding the outer irregular surface of the die (as made) to a suitable shape for a press fit would be economically prohibitive.

Improved wire drawing die construction, particularly for drawing wires of strong, hard metals, such as tungsten, molybdenum, steel etc. at elevated temperatures would be of considerable benefit to the art.

SUMMARY OF THE INVENTION This invention presents such improved wire drawing die construction. In its simplest form, the composite die construction of this invention consists of an outer metal bonded carbide jacket enclosing a core made of crystalline material selected from the group consisting of diamond, cubic boron nitride and polycrystalline mixtures thereof, the core having a centrally located hole extending completely therethrough and functioning as the strand-shaping/sizing element of the die. The metal bonded carbide is directly bonded to the core, adds strength thereto and is easily shaped in the form of a solid of revolution. 1n the preferred construction of this invention (for drawing wire of 0.008 inch diameter or larger), a composite assembly is employed in which at least one high strength metal ring is press fitted around a composite structure comprising a polycrystalline diamond core within a metal bonded carbide jacket. This arrangement pennanently places the outer surface of revolution of the composite structure under significant compressive stress (greater than about 10,000 psi).

BRIEF DESCRIPTION OF THE DRAWING This invention and the objects and advantages thereof will be better understood from the description embodied in this specification and in the drawings in which:

FIG. I is a sectional view through a composite wire drawing die in which a polycrystalline core is in a generally cylindrical configuration and has a doubletapered hole extending therethrough; the metal bonded carbide jacket is directly bonded thereto;

FIG. 2 is a section through a composite die in the shape of a solid of revolution composed of an inner polycrystalline layer flanked top and bottom by layers of metal bonded carbide (and girded by an integral col lar of metal bonded carbide) directly bonded thereto; the polycrystalline mass defines the throat region at least of the double-tapered hole employed to conduct the wire shaping and sizing;

FIG. 3 is a sectional view partially in elevation illustrating an exemplary high pressure, high temperature apparatus for the preparation of the composite structures of this invention;

FIG. 4 is a sectional view illustrating a charge assembly for introduction into the working volume of the apparatus of FIG. 3 and FIGS. 5 and 6 illustrate embodiments of composite die/compression ring assemblies.

DESCRIPTION OF THE PREFERRED EMBODIMENT The tendency for diamond dies, particularly those for drawing larger (i.e. 0.013 inch diameter and up) wires to burst in use before they have reached the stage at which they need replacement due to wear has been known for a long time. It has been recognized that this type of failure is due to lack of sufficient support of the diamond, which is weak in tension. However, the selection of supporting metal matrices has been limited by the criteria discussed hereinabove and as a consequence, no construction has as yet been devised able to apply significant compressive stress to the outer surface of the single diamond, particularly during hot operation.

The critical features of the instant invention are: first, the identification of an ideal support material for the die core that can actually add to the strength of the die core, is exceedingly stiff and, as well, can be readily fashioned into the shape of a solid of revolution to permit mounting of the composite within strong binding rings. Such a support material must be able to accept large compressive forces from such binding rings and to transmit these large compressive stresses to the die core in order to offset the application of tensile forces thereto during the wire drawing process. Next, having selected the ideal support material, it becomes necessary to devise the manner by which this support material can be interposed between the die core (polycrystalline or single crystal) and the binding ring construction whereby significant compressive stress will be permanently applied to the outer surface of the die core.

The ideal support material has been determined to be metal bonded carbide (also referred to as sintered carbide and cemented carbide) and it has been found that the best way to interpose this material between the die core and the binding ring(s) is to bond the metal bonded carbide directly to the die core. In the preferred construction the metal bonded carbide is in the form of a jacket properly shaped at its outer surface to match the inner surface of the binding ring in which the composite is to be mounted. In so directly bonding the die core and the metal bonded carbide it was found that the unique interface created between these materials by the application of high pressures and high temperatures acts to compensate for differences in the thermal expansion coefficients of the core and carbide materials. This latter aspect is important both during manufacture of the composite die and during use of the die in the heated condition.

The preferred construction for the composite is shown in FIG. 1. The die core 11 in a generally cylindrical shape has a properly sized and shaped hole 12 therethrough. Core 11 is shown as a polycrystalline mass of diamond crystals, cubic boron nitride crystals or a mixture thereof, however, a single crystal diamond could be used. Jacket 13 is a mass of metal bonded carbide directly bonded to die core 11 along an interface free of voids and irregular and interlocked on the scale of about l-lOO micrometers, the interlocking occurring between individual abrasive crystals and portions of the metal bonded carbide mass. This interface structure develops both with polycrystalline and single crystal diamond. With the latter microscopic etching of the diamond occurs and the etched regions are filled with the metal bonded carbide.

An alternate construction is shown in FIG. 2 in which the composite consists of an inner polycrystalline abrasive layer 21 initially flanked on top, bottom and side by masses 22a and 22b and girded by an annulus 220 of metal bonded carbide. In the completed composite shown, masses 22a, 22b and 220 are integral. In both constructions the composites have been shaped as solids of revolution (preferably with a 24 taper). In this way the throat of hole 23 is made of the strong highly wear-resistant material.

One preferred form of a high pressure, high temperature apparatus in which the composites l0 and 20 may be prepared is the subject of [1.8. Pat. No. 2,941,248 Hall (incorporated by reference) and is briefly illustrated in FIG. 3. A charge assembly useful in the practice of this invention is shown in FIG. 4.

Apparatus 30 includes a pair of cemented tungsten carbide punches 31 and 31' and an intermediate belt or die member 32 of the same material. Die member 32 includes an aperture 33 in which there is positioned a reaction vessel 34 shaped to contain a charge assembly to be described hereinbelow. Between punch 31 and die 32 and between punch 31' and die 32 there are included gasket/insulating assemblies 35 each comprising a pair of thermally insulating and electrically nonconducting pyrophyllite members 36 and 37 and an intermediate metallic gasket 38.

Reaction vessel 34 in one preferred form includes a hollow salt cylinder 39. Cylinder 39 may be of another material, such as talc, which (a) is not converted during high pressure-high temperature operation to a stronger, stiffer state (as by phase transformation and/or compaction) and (b) is substantially free of volume discontinuities occurring under the application of high temperatures and pressures, as occurs, for example with pyrophyllite and porous alumina. Materials meeting the criteria set forth in US. Pat. No. 3,030,662 Strong (column 1, line 59 through column 2, line 2, incorporated by reference) are useful for preparing cylinder 39.

Positioned concentrically within and adjacent cylinder 39 is a graphite electrical resistance heater tube 40. Within graphite heater tube 40 there is in turn concentrically positioned the cylindrical salt liner 41. The ends of liner 41 are fitted with salt plugs 42, 42', disposed at the top and bottom, respectively.

Electrically conductive metal end disks 43 and 43' are utilized at each end of cylinder 39 to provide electrical connection to graphite heater tube 40. Adjacent each disk 43, 43 is an end cap assembly 44 and 44' each of which comprises a pyrophyllite plug or disk 45 surrounded by an electrically conducting ring 46.

Operational techniques for simultaneously applying both high pressures and high temperatures in this apparatus are well known to those skilled in the superpressure art. The foregoing description relates to merely one high pressure, high temperature apparatus. Various other apparatuses are capable of providing the required pressures and temperature that may be employed within the scope of this invention.

Charge assembly although not illustrated to the same scale, fits within space 51 of the apparatus of FIG. 3. Charge assembly 50 consists of cylindrical sleeve 52 of shield metal selected from the group consisting of zirconium, titanium, tantalum, tungsten and molybdenum. Within cylindrical shield metal sleeve 52 is a subassembly confined within shield metal disk 54 and shield metal cup 56. For the arrangement shown, which will produce a composite having a straight hole predisposed through a polycrystalline core, a wire 57 of appropriate dimension (e.g. a 0.010 inch diameter tungsten wire) is properly located and supported by attachment thereof, as by welding, to the bottom of cup 56. A mass 58 of strong abrasive grains (diamond, cubic boron nitride or a mixture thereof) is disposed around wire 57 to fill the cavity in sleeve 59 made of coldpressed sinterable carbide powder (mixture of carbide powder and appropriate metal bonding medium therefor). If desired, sleeve 59 may be made of presintered metal bonded carbide as will be described hereinbelow.

Tungsten is a particularly good metal to use for the formation of the hole to pass through the polycrystalline core, because tungsten has a high melting point and is a stiff enough metal to resist distortion by the abrasive grains during the compression and sintering step at the high temperatures and pressures employed. Tungsten is also not too difficult to dissolve or grind away later. Other materials may also be employed e.g., molybdenum, zirconium, titanium, tantalum, rubidum, rhodium, rhenium, osmium or refractory carbides, and even non-metals such as refractory oxides. The wire need not have a uniform cross section as shown, but may be of a configuration such as will minimize the effort required to shape the pre-formed hole to the desired double taper.

The balance of the volume in charge assembly 59 is taken up with disk 61a, 6112 made of the same material as cylinder 39, e.g., sodium chloride, and disks 62a, 62b made of hexagonal boron nitride. Disks 62a, 6212 are provided to minimize the entry of undesirable substances into the sub-assembly defined by disk 54 and cup 56. It has been found that when either zirconium or titanium is employed for sleeve 52, disk 54 and cup 56, the presence of these materials enhances the sintering of the abrasive grains and bonding of the abrasive grain mass to the metal bonded carbide jacket.

EXAMPLE 1 Jacket 59 was made from cold pressed Carboloy grade 55A powder (13 wt. Co, 87 wt. WC) with a 0.101 inch diameter hole bored into it. This sleeve was set into a zirconium cup, which had a 0.010 inch diameter tungsten wire welded to the bottom thereof and disposed to extend vertically in the center of the cup as shown in FIG. 4. 100/200 mesh size diamond grit was poured in around the tungsten wire to till the hole in the sinterable carbide sleeve. Zirconium metal (0.002 inch thick) was utilized to fashion the sleeve 52 and covers 54, 63a and 631). This charge assembly was introduced into the high pressure, high temperature apparatus shown in FIG. 2 and, after exposure thereof to approximately 55 kilobars of pressure and to a temperature of I500C for about 60 minutes, the temperature was reduced to near room temperature and then the pressure was released. The diamond/metal bonded carbide composite recovered from the apparatus was about 0.23 inch in diameter and about 0.22 inch long. The tungsten wire, the zirconium outer jacket and some of the metal bonded carbide outer surface were dissolved away in a bath of hot HF HNO- Thereafter, a coating of molten polyethylene was applied over the surface of the composite for protection, while further etching was in progress in order to completely remove the tungsten wire.

In those constructions in which diamond crystals are employed for the mass 58, very extensive diamond-todiamond bonding is achieved as is described in the above-identified application Ser. No. 212,408. When CBN crystals or a mixture of CBN crystals and dia mond crystals are employed, a metallic phase must be included therewith containing aluminum atoms and atoms of at least one alloying element selected from the group consisting of nickel, cobalt, manganese, iron, vanadium and chromium. The amount of aluminum present relative to the amount of alloying metal is not critical and may range from about equal parts by weight to about 1 part of aluminum to parts of alloying metal.

The amount of aluminum in the starting material may range from about I to about 40 percent by weight of the CBN, while the range of the alloying metal may range from about 2 to about 100 percent by weight of the CBN. The amount of these alloying metals remaining in the consolidated CBN as matrix material will vary depending upon the pressure and length of application of high pressure/high temperature conditions. In any event the quantity of aluminum plus alloying metal atoms in the compacted CBN will be in excess of about 1 percent by weight of the CBN.

The preferred size range for the diamond grains is 230-270 mesh (U.S. Sieve size) and for the CBN is 0.1-l0 micrometers. Other sizes may, of course, be employed. The diamond grains may range in size from about 0.1 micrometers to about 500 micrometers in largest dimension and the CBN grains may range from about 0.1 to 20 micrometers in largest dimensions.

The preferred starting content for the die core when diamond grains are employed is I00 percent volume diamond resulting in a composition for the die core, when formed, of 90-98 percent by volume diamond and 2-10 percent by volume of the metal bonding medium employed for the metal bonded carbide. In any event in the completed die core there must be a concentration of diamond therein greater than percent by volume to insure diamond-to-diamond bonding. In the case of starting diamond concentrations ranging 70-90 percent by volume, sinterable carbide powder or metal powders may be mixed with the diamond.

The preferred starting content for the die core when CBN grains are employed is -97 percent by volume CBN with the balance being the metallic medium. The composition of the completed die core will contain the CBN content, metallic medium present as various pha ses and some of the metal bonding medium from the metal bonded carbide.

In the preparation of a composite die having a diamond core the charge assembly 50 is placed in the apparatus 30, pressure is applied thereto and the system is then heated. The temperatures employed are in the range from about I,300l ,600 C for periods of time in excess of about 3 minutes in order to sinter the carbide/metal bonding agent mixture while at the same time the system is subjected to very high pressure, e.g., of the order of 55 kilobars, to insure thermodynamically stable conditions for the diamond content of the system. At 1,300 C the minimum pressure should be about 50 kilobars and at l,400 C the minimum pressure should be about 52.5 kilobars. At the temperatures employed, of course, the metal bonding component of the system is melted making some of the metal bonding component available for displacement from mass 59 into mass 58, where it must be able to function as a catalyst-solvent for diamond growth, particularly in the preparation of a polycrystalline diamond core.

In the preparation of a composite die having a CBN or CBN/diamond core the charge assembly 50 is placed in the apparatus 30, pressure is applied thereto and the system is then heated. The temperatures employed are in the range from about l,300l,600 C for periods of time in excess of about 3 minutes while at the same time the system is subjected to very high pressure e.g., of the order of 55 kilobars to insure thermodynamically stable conditions for the CBN content of the system. At 1,300C the minimum pressure should be about 40 kilobars and at I,600C the minimum pressure should be about 50 kilobars. At the temperatures employed the sintering agent in mass 59 is melted, making cobalt, nickel or iron (depending on the particular sinterable carbide formula) available for displacement from mass 59 into mass 58, where it alloys with the molten aluminum alloy, which is present or formed in mass 58. The metallic medium so formed functions as an effective bonding agent for the CBN crystals near the interface between mass 58 and 59 for bonding these crystals to each other and to the sintered carbide. The rest of the crystals in the mass of CBN are bonded together by the metallic medium present (introduced or formed in situ) and by reaction of this alloy with CBN.

The direct bonding relationship created between the very high strength wear resistant core and the surrounding or flanking stiff carbide support material obviates any need for the interposition of any bonding layer therebetween. By providing stiff, non-yielding support material in direct contact with the inner die core (e.g., mass 11 or mass 21) a composite results which is unusually strong and durable, because of the complementary nature of the properties of these materials used in combination in this application. The quality of the bond at the interface is such that the interface is in general stronger than the tensile strength of th abrasive grains.

The carbide powder, where employed, is preferably a tungsten carbide molding powder (mixture of tungsten carbide powder and cobalt powder) commercially available in particle sizes of from 1 to 5 microns. The tungsten carbide may, if desired, be replaced in whole or in part by either or both of titanium carbide and tantalum carbide. Also, small quantities of other carbide powders may be utilized in order to secure unusual properties in the composite. Since some use of nickel and iron has been made in the bonding of carbides, the material for providing the metal bond in the cemented carbide may be selected from the group consisting of cobalt, nickel, iron and mixtures thereof. Cobalt, however, is preferred as the metal bond material. The composition of carbide molding powders useful in the prac tice of this invention may consist of mixtures containing about 75-94 percent by weight carbide and about 6-25 percent by weight metal bond material. Examples of carbide powders used are Carboloy grade 883 carbide (6% by weight Co, 94% by weight WC) and Carboloy grade 55A (13% by weight Co, 87% by weight WC). A presintered cemented carbide sleeve (FIG. 1) or disks (FIG. 2) can be prepared, if desired using the above described powders. The sintered component is then used in place of the cold-pressed shapes referred to hereinabove.

Composite dies may, of course, be prepared with no hole therethrough, with a straight hole therethrough or with a double tapered hole therethrough, but in any event some shaping of the hole will be required to provide the exact dimensions. Shaping is facilitated by having a hole built-into the composite die core so that a wire impregnated with diamond dust can be drawn therethrough. If desired, an initial hole can be made through the die core using a laser. When the holes in die cores become enlarged from normal wear and ero sion, the holes can be re-shaped for drawing larger wire.

After composites (e.g. composites and have been produced with accurately dimensioned holes therethrough, one of the prime advantages of this construction is made use of by accurately shaping the outer surface of the composite into the shape of a solid of revolution (e.g. right cylinder, truncated cone). When properly shaped substantially concentric with the hole through the die core, the composite can be properly received within one or more high strength binding rings whereby compressive stress in excess of 100,000 psi has been uniformly applied thereto. When properly de signed and assembled, this compressive stress application will be permanently and uniformly transmitted through the composite to the outer surface of the core therethrough.

Support rings may be made of suitable high strength (under the operating conditions) material, such as superalloys, stainless steel, high strength steel dispersionhardened alloys, reinforced metals and plastics and cemented carbides.

In the assembly shown in FIG. 5, the carbide jacket of composite body has been accurately formed in the shape of a right circular cylinder. Provisions are made for a press fit of composite 70 by force fitting this body (e.g., 0.3020 inch O.D.) intometal ring 71 (e.g., 0.3017 inch l.D.), which has its outer surface tapered (preferably 2-4 percent) to match the inside tapered surface of metal ring 72 into which the sub-assembly of elements 70 and 71 are pressed. Safety ring 73 may be provided to contain the die assembly in case of burstmg.

For a die assembly for drawing tungsten wire, for example, ring 71 is made of H21 steel, ring 72 is made of a superalloy (Rent? 41) and ring 73 is made of stainless steel. For a die assembly for drawing a softer material at lower temperatures, e.g., copper wire, all of the rings may be made of stainless steel.

The die assembly shown in FIG. 6 is easier to assemble and requires fewer parts. In this embodiment the composite die has the outer surface of its cemented carbide jacket accurately ground in the shape of a truncated cone having a taper of about 2-4 percent. This tapered composite is press fitted into ring 81. As in FIG. 5, ring 82 is provided for safety, but the compressive forces are provided by ring 81.

Die assemblies in which a cylindrically shaped composite die are shrunk fit into a support ring may also be made. These assemblies are useful for low (below about C) temperature wire drawing, e.g., copper. Such assemblies are much more limited as to the amount of compressive stress that can be applied to the composite die.

The holes through the die core need not be circular, of course. Also, the diamond core may be a single diamond crystal, rather than a polycrystalline diamond mass as long as the objective of providing considerable hoop compression to the diamond die is attained and the bursting effect of the wire being drawn through the die is actually carried more by the binding rings, which are strong in tension and less by the diamond, which is weak in tension.

EXAMPLE 2 A thick-walled sleeve having a length of about 0.15 inch, a bore of about 0.10 inch and an outside diameter of 0.25 inch was made from a cold-pressed mass of powder containing 87% WC and 13% Co by weight. This sleeve was placed inside a closely fitted zirconium cup having a wall thickness of about 0.002 inch and then the central hole was filled with 230/270 mesh syntamped into place. Next, two disks of zirconium, each about 0.25 inch in diameter and 0.002 inch thick, were placed on the top end of the sleeve full of diamond. The zirconium cup and the pressed powder sleeve were all contained inside a zirconium tube having a wall thickness of 0.001 inch. This assembly was placed in a pressed salt holder, which fitted inside a graphite heater tube, as described in connection with FIG. 3. After raising the pressure on the specimen to about 55 kilobars, it was heated to about l,550C for 60 minutes. After cooling and subsequent pressure reduction, the cup, sleeve and diamond combination was recovered as a strong cylinder. The adherent zirconium was dissolved in an HF-l-lNO mixture and one circular face of the cylinder was polished on a diamond lap for examination of an end of the central column of diamond. Extensive diamond-to-diamond bonding was observed. The outer cylindrical surface (the cemented carbide) was then ground to a diameter of 0.204 inches. A soft steel ring was made with an inner diameter of 0.2024 inches, an outer diameter of 1.50 inches and a thickness of 0.50 inch, and heated to 400C. The diamondbearing cylinder described above was then quickly driven into the hole in the steel ring and the assembly was allowed to cool. Finally, a 0.015 inch hole suitable for drawing tungsten wire was produced in the diamond column and the complete die assembly was used to draw tungsten wire. In the wire drawing process the die was kept at about 400C and the wire was pre-heate to about 800C.

After about 54 kilograms of tungsten has passed through the die, the outgoing wire size and shape indicated that the diamond portion of the die had cracked due to bursting forces exerted by the wire passing through it. Evidently the radial compressive support provided by the soft steel ring was insufficient at the operating conditions. However, the life of the die was comparable to that shown by natural single crystal diamond for these conditions.

EXAMPLE 3 A composite assembly of diamond grit in a WC +Co sleeve similar to that described in Example 2 was prepared, except that the hole in the sleeve was 0.125 inch in diameter. After encapsulation in zirconium, exposure to high pressure and temperature, as described in Example 2, the composite cylinder was recovered and ground to an outside diameter of 0.204 inch and pressed into a soft steel sleeve as in Example 2.

The diamond column was pierced by standard drilling techniques to prepare a wire drawing die for drawing 0.0128 inch diameter tungsten wire and the die was put into service under conditions similar to those of Example 2.

After about 700 kilograms of tungsten wire had been drawn through this die, the inner cylinder became loose in the steel ring and the die was retired from service in order to be examined. It was found that the die had not cracked nor was it appreciably worn, but its looseness in the steel support ring made it impractical to use. Even so, this die had given over twice the useful life of a conventional single diamond crystal die. It was then re-mounted in a ring, which gave it more external compressive support and bored to draw larger sizes of wire, after which it performed satisfactorily.

EXAMPLE 4 A composite die kernel consisting of polycrystalline diamond in a sintered tungsten carbide-cobalt sleeve was prepared as in Example 3 and ground to an outside diameter of 0.204 inch. Then this kernel was pressed into a hardened and ground sleeve made of hot work tungsten steel, which had an inside diameter of 0.2037 inch, a length of 0.250 inch and whose outside wall was tapered (a diameter of 0.450 inch at one end and 0.445

inch at the other). This assembly was then pressed with a force of about 3000 pounds into a tapered hole in a Rene 41 ring, which had a thickness of 0.50 inch and an outside diameter (including a 0.062 inch wall of 18-8 stainless steel guard ring) of 1.50 inch. The Rene ring thus exerted a compressive stress of about 120,000 psi on the assembly inside it to resist the bursting forces developed by the wire passing through the die. The completed assembly was similar to that shown in FIG. 5.

The diamond core was pierced and finished by conventional methods to make a die suitable for drawing 0.013 inch diameter tungsten wire. After several months of use, over 1100 kilograms of hot tungsten wire was drawn through this die in normal production. The die appeared to be as good as new and continued in service. Up to this point it has produced several times as much tungsten wire as even the best conventional diamond dies used under the same operating conditions.

EXAMPLE 5' A hole 0.170 inch in diameter was made in a cylinder of sintered tungsten carbide-cobalt (87% wt. WC, 13 wt. Co) which had an outside diameter of 0.347 inch and a length of about 0.250 inch. The hole was filled with 230/270 mesh synthetic diamond grit and the assembly was enclosed in 0.002 inch thick zirconium sheet and was placed in a high pressure, high temperature reaction cell as described hereinabove. The charge assembly was exposed to a pressure of about 55 kilobars, while it was heated to about 1550C for 58 minutes. After cooling, the pressure was removed and the mass was recovered as a strong cylinder. The zirconium outer layer was removed with abrasive and each face of the cylinder was polished on a diamond lap until the ends of the diamond core were flat and could be observed under the microscope. The diamond core was found to consist of many grains firmly bonded together with much diamond-to-diamond bonding evident. The length of the cylinder was 0.205 inches. The sides of .the cylinder were then ground with a 2 percent taper so that the large end had a diameter of 0.329 inch and the small end had a diameter of 0.325 inch.

A ring was made of 18-8 stainless steel having a thickness of 0.375 inch, an outer diameter of 1.00 inch and an inner hole with a 2 percent having a diameter of 0.3266 inch at the large end. The diamond-carbide composite cylinder was pushed into this hole with a force of about 500 pounds so that the completed assembly appeared as depicted in FIG. 6, except for the safety ring. In this way the steel outer ring exerted a hoop confining stress of about 40,000 psi on the composite cylinder inside it.

The diamond core in this assembly was pierced and finished by conventional means to produce a die for drawing 0.403 inch diameter copper wire. When the die had been in service for several months over 50,000 pounds of copper wire and had been produced with an excellent surface finish suitable for insulating enamel. in addition, the polycrystalline rubbing surface appears to improve the lubrication of the wire passing through it by trapping lubricant from the incoming wire. This die assembly was continued in use.

EXAMPLE 6 A central hole 0.125 inch in diameter was made in a cylinder of cemented tungsten carbide, containing 87% WC and 13% Co by weight, having an outside diameter of 0.347 inch and a length 0.125 inch. This sleeve was placed in a zirconium cup having a wall thickness of 0.002 inch and the central hole was filled with a mixture of 94 volume per cent cubic boron nitride powder having particle sizes between 0.1 and 10 micrometers and 6 volume percent of 300/400 mesh NiAl powder. Zirconium disks were placed on top of the cylinder in the cup and the assembly was placed in a high pressure, high temperature reaction vessel as described hereinabove and was exposed to a pressure of about 55 kilobars and then heated to about l,550C for 54 minutes. After cooling, the pressure was released and the composite cylinder was recovered. One end of the cylinder was polished on a diamond lap to expose the cubic boron nitride column. The bonding between grains of the cubic boron nitride and also to the sintered carbide sleeve appeared to be excellent when examined in a microscope at 300x. This cylinder, which is yet to be tested, is expected to be particularly suited to include the drawing of steel or tungsten wire, where the lower chemical reactivity of cubic boron nitride (compared with diamond) may be'advantageous.

EXAMPLE 7 A hole about 0.140 inch in diameter was formed in a cylinder made of sintered tungsten carbide containing 87 wt. WC and 13 wt. Co. The cylinder had an outside diameter of 0.347 inch and a length of 0.125 inch. It was placed in a zirconium cup having a well thickness of 0.002 inch and a layer of powdered tungsten carbide-cobalt mixture (87 wt. WC, 13 wt. Co) was lightly tamped in the bottom of the hole. Next, a single crystal natural diamond, suitable for a wire drawing die, about 0.070 inch thick and 0.13 inch in average diameter, was placed on the tamped mixture and more powdered metal mixture was placed around and above the diamond crystal and pressed into place with an average pressure of about 10,000 psi, using a small hand press. The completed charge assembly was enclosed in 0.002 inch zirconium sheet and placed in a high pressure, high temperature reaction vessel as described hereinabove. The charge assembly was then ex posed to a pressure of about 55 kilobars and a temperature of about 1550C for about one hour. After cooling to 30C, the pressure was removed and the composite cylinder was recovered. The adhering zirconium and the sintered metal covering the top and bottom faces of the diamond crystal were removed with a fine jet of abrasive grit. The partly exposed diamond appeared to be clear and intact although its surface was lightly frosted indicating the etched surface resulting from a slight reaction with the metal around it. Thickness measurements on the buried diamond indicated that the amount of diamond lost was negligible.

The outer wall of the composite cylinder was then ground to a 2 percent taper so that one end had a diameter of 0.2525 inch and the other end had a diameter of 0.250 inch. The cylinder was then pushed into a correspondingly tapered hole in a Rent: 41 ring, which had a thickness of 0.312 inch and an outside diameter of 0.88 inch. The Rene: 41 ring was fitted snugly into a stainless steel guard ring of 1.00 inch outside diameter. About 1700 pounds of force were used to push the tapered cylinder into the Rene 41 ring and the confining pressure thereby developed on the cylinder was estimated to be about 120,000 psi. The completed assembly appeared as depicted in FIG. 6.

The complete assembly, along with several others of similar construction, after piercing and finishing of the resulting hole, is to be used for drawing tungsten wire in the 0.007 to 0.012 inch diameter range.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. A composite wire drawing die comprising in combination:

a. an inner mass having a centrally-located double tapered hole extending therethrough, said inner mass being predominately of a material selected from the group consisting of polycrystalline diamond, polycrystalline cubic boron nitride plus a metallic phase containing aluminum atoms and mixtures thereof;

at least one mass of metal bonded carbide directly bonded to and girding said inner mass for symmetrical support thereof, said mass of metal bonded carbide being predominately of carbide material selected from the group consisting of tungsten carbide, titanium carbide, tantalum carbide and mixtures thereof with the metal bonding material therein being present in a quantity of from about 6 percent to about 25 percent by weight and being selected from the group consisting of cobalt, nickel, iron and mixtures thereof,

c. the interface between said inner mass and said at least one mass of metal bonded carbide being free of voids and being irregular and interlocking on the scale of lmicrometers.

2. The wire drawing die as recited in claim 1 wherein the inner mass is predominately polycrystalline diamond.

3. The wire drawing die as recited in claim 1 wherein the inner mass is predominately polycrystalline cubic boron nitride.

4. The wire drawing die as recited in claim 1 wherein the inner mass is in a generally cylindrical form and the carbide mass is in the form of a jacket therearound.

5. The wire drawing die as recited in claim 4 wherein the composite is in the shape of a truncated cone.

6. The wire drawing die as recited in claim 1 wherein the composite is press fitted into a high tensile strength annular body.

7. The wire drawing die as recited in claim 6 wherein the outer wall of the composite is in a state of permanent compressive stress of at least about 10,000 psi.

8. The wire drawing die as recited in claim 6 wherein the annular body is made of metal.

9. The wire drawing die as recited in claim 1 wherein the composite is in the shape of a truncated cone.

10. The wire drawing die as recited in claim 1 wherein the inner mass is both flanked and girded by the mass of metal bonded carbide. 

2. The wire drawing die as recited in claim 1 wherein the inner mass is predominately polycrystalline diamond.
 3. The wire drawing die as recited in claim 1 wherein the inner mass is predominately polycrystalline cubic boron nitride.
 4. The wire drawing die as recited in claim 1 wherein the inner mass is in a generally cylindrical form and the carbide mass is in the form of a jacket therearound.
 5. The wire drawing die as recited in claim 4 wherein the composite is in the shape of a truncated cone.
 6. The wire drawing die as recited in claim 1 wherein the composite is press fitted into a high tensile strength annular body.
 7. The wire drawing die as recited in claim 6 wherein the outer wall of the composite is in a state of permanent compressive stress of at least about 10,000 psi.
 8. The wire drawing die as recited in claim 6 wherein the annular body is made of metal.
 9. The wire drawing die as recited in claim 1 wherein the composite is in the shape of a truncated cone.
 10. The wire drawing die as recited in claim 1 wherein the inner mass is both flanked and girded by the mass of metal bonded carbide. 